'\" t
.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\" and Copyright (c) 2002, 2006, 2020 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified Sat Jul 24 17:34:08 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Sun Jan  7 01:41:27 1996 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Sun Apr 14 12:02:29 1996 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Sat Nov 13 16:28:23 1999 by Andries Brouwer (aeb@cwi.nl)
.\" Modified 10 Apr 2002, by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified  7 Jun 2002, by Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Added information on real-time signals
.\" Modified 13 Jun 2002, by Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Noted that SIGSTKFLT is in fact unused
.\" 2004-12-03, Modified mtk, added notes on RLIMIT_SIGPENDING
.\" 2006-04-24, mtk, Added text on changing signal dispositions,
.\"		signal mask, and pending signals.
.\" 2008-07-04, mtk:
.\"     Added section on system call restarting (SA_RESTART)
.\"     Added section on stop/cont signals interrupting syscalls.
.\" 2008-10-05, mtk: various additions
.\"
.TH signal 7 2024-06-17 "Linux man-pages 6.9.1"
.SH NAME
signal \- overview of signals
.SH DESCRIPTION
Linux supports both POSIX reliable signals (hereinafter
"standard signals") and POSIX real-time signals.
.SS Signal dispositions
Each signal has a current
.IR disposition ,
which determines how the process behaves when it is delivered
the signal.
.P
The entries in the "Action" column of the table below specify
the default disposition for each signal, as follows:
.TP
Term
Default action is to terminate the process.
.TP
Ign
Default action is to ignore the signal.
.TP
Core
Default action is to terminate the process and dump core (see
.BR core (5)).
.TP
Stop
Default action is to stop the process.
.TP
Cont
Default action is to continue the process if it is currently stopped.
.P
A process can change the disposition of a signal using
.BR sigaction (2)
or
.BR signal (2).
(The latter is less portable when establishing a signal handler;
see
.BR signal (2)
for details.)
Using these system calls, a process can elect one of the
following behaviors to occur on delivery of the signal:
perform the default action; ignore the signal;
or catch the signal with a
.IR "signal handler" ,
a programmer-defined function that is automatically invoked
when the signal is delivered.
.P
By default, a signal handler is invoked on the
normal process stack.
It is possible to arrange that the signal handler
uses an alternate stack; see
.BR sigaltstack (2)
for a discussion of how to do this and when it might be useful.
.P
The signal disposition is a per-process attribute:
in a multithreaded application, the disposition of a
particular signal is the same for all threads.
.P
A child created via
.BR fork (2)
inherits a copy of its parent's signal dispositions.
During an
.BR execve (2),
the dispositions of handled signals are reset to the default;
the dispositions of ignored signals are left unchanged.
.SS Sending a signal
The following system calls and library functions allow
the caller to send a signal:
.TP
.BR raise (3)
Sends a signal to the calling thread.
.TP
.BR kill (2)
Sends a signal to a specified process,
to all members of a specified process group,
or to all processes on the system.
.TP
.BR pidfd_send_signal (2)
Sends a signal to a process identified by a PID file descriptor.
.TP
.BR killpg (3)
Sends a signal to all of the members of a specified process group.
.TP
.BR pthread_kill (3)
Sends a signal to a specified POSIX thread in the same process as
the caller.
.TP
.BR tgkill (2)
Sends a signal to a specified thread within a specific process.
(This is the system call used to implement
.BR pthread_kill (3).)
.TP
.BR sigqueue (3)
Sends a real-time signal with accompanying data to a specified process.
.SS Waiting for a signal to be caught
The following system calls suspend execution of the calling
thread until a signal is caught
(or an unhandled signal terminates the process):
.TP
.BR pause (2)
Suspends execution until any signal is caught.
.TP
.BR sigsuspend (2)
Temporarily changes the signal mask (see below) and suspends
execution until one of the unmasked signals is caught.
.\"
.SS Synchronously accepting a signal
Rather than asynchronously catching a signal via a signal handler,
it is possible to synchronously accept the signal, that is,
to block execution until the signal is delivered,
at which point the kernel returns information about the
signal to the caller.
There are two general ways to do this:
.IP \[bu] 3
.BR sigwaitinfo (2),
.BR sigtimedwait (2),
and
.BR sigwait (3)
suspend execution until one of the signals in a specified
set is delivered.
Each of these calls returns information about the delivered signal.
.IP \[bu]
.BR signalfd (2)
returns a file descriptor that can be used to read information
about signals that are delivered to the caller.
Each
.BR read (2)
from this file descriptor blocks until one of the signals
in the set specified in the
.BR signalfd (2)
call is delivered to the caller.
The buffer returned by
.BR read (2)
contains a structure describing the signal.
.SS Signal mask and pending signals
A signal may be
.IR blocked ,
which means that it will not be delivered until it is later unblocked.
Between the time when it is generated and when it is delivered
a signal is said to be
.IR pending .
.P
Each thread in a process has an independent
.IR "signal mask" ,
which indicates the set of signals that the thread is currently blocking.
A thread can manipulate its signal mask using
.BR pthread_sigmask (3).
In a traditional single-threaded application,
.BR sigprocmask (2)
can be used to manipulate the signal mask.
.P
A child created via
.BR fork (2)
inherits a copy of its parent's signal mask;
the signal mask is preserved across
.BR execve (2).
.P
A signal may be process-directed or thread-directed.
A process-directed signal is one that is targeted at (and thus pending for)
the process as a whole.
A signal may be process-directed
because it was generated by the kernel for reasons
other than a hardware exception, or because it was sent using
.BR kill (2)
or
.BR sigqueue (3).
A thread-directed signal is one that is targeted at a specific thread.
A signal may be thread-directed because it was generated as a consequence
of executing a specific machine-language instruction
that triggered a hardware exception (e.g.,
.B SIGSEGV
for an invalid memory access, or
.B SIGFPE
for a math error), or because it was
targeted at a specific thread using
interfaces such as
.BR tgkill (2)
or
.BR pthread_kill (3).
.P
A process-directed signal may be delivered to any one of the
threads that does not currently have the signal blocked.
.\" Joseph C. Sible notes:
.\" On Linux, if the main thread has the signal unblocked, then the kernel
.\" will always deliver the signal there, citing this kernel code
.\"
.\"     Per this comment in kernel/signal.c since time immemorial:
.\"
.\"     /*
.\"     * Now find a thread we can wake up to take the signal off the queue.
.\"     *
.\"     * If the main thread wants the signal, it gets first crack.
.\"     * Probably the least surprising to the average bear.
.\"     */
.\"
.\" But this does not mean the signal will be delivered only in the
.\" main thread, since if a handler is already executing in the main thread
.\" (and thus the signal is blocked in that thread), then a further
.\" might be delivered in a different thread.
.\"
If more than one of the threads has the signal unblocked, then the
kernel chooses an arbitrary thread to which to deliver the signal.
.P
A thread can obtain the set of signals that it currently has pending
using
.BR sigpending (2).
This set will consist of the union of the set of pending
process-directed signals and the set of signals pending for
the calling thread.
.P
A child created via
.BR fork (2)
initially has an empty pending signal set;
the pending signal set is preserved across an
.BR execve (2).
.\"
.SS Execution of signal handlers
Whenever there is a transition from kernel-mode to user-mode execution
(e.g., on return from a system call or scheduling of a thread onto the CPU),
the kernel checks whether there is a pending unblocked signal
for which the process has established a signal handler.
If there is such a pending signal, the following steps occur:
.IP (1) 5
The kernel performs the necessary preparatory steps for execution of
the signal handler:
.RS
.IP (1.1) 7
The signal is removed from the set of pending signals.
.IP (1.2)
If the signal handler was installed by a call to
.BR sigaction (2)
that specified the
.B SA_ONSTACK
flag and the thread has defined an alternate signal stack (using
.BR sigaltstack (2)),
then that stack is installed.
.IP (1.3)
Various pieces of signal-related context are saved
into a special frame that is created on the stack.
The saved information includes:
.RS
.IP \[bu] 3
the program counter register
(i.e., the address of the next instruction in the main program that
should be executed when the signal handler returns);
.IP \[bu]
architecture-specific register state required for resuming the
interrupted program;
.IP \[bu]
the thread's current signal mask;
.IP \[bu]
the thread's alternate signal stack settings.
.RE
.IP
If the signal handler was installed using the
.BR sigaction (2)
.B SA_SIGINFO
flag, then the above information is accessible via the
.I ucontext_t
object that is pointed to by the third argument of the signal handler.
This object reflects the state at which the signal is delivered,
rather than in the handler;
for example,
the mask of blocked signals stored in this object
will not contain the mask of new signals blocked through
.BR sigaction (2).
.IP (1.4)
Any signals specified in
.I act\->sa_mask
when registering the handler with
.BR sigaction (2)
are added to the thread's signal mask.
The signal being delivered is also
added to the signal mask, unless
.B SA_NODEFER
was specified when registering the handler.
These signals are thus blocked while the handler executes.
.RE
.IP (2)
The kernel constructs a frame for the signal handler on the stack.
The kernel sets the program counter for the thread to point to the first
instruction of the signal handler function,
and configures the return address for that function to point to a piece
of user-space code known as the signal trampoline (described in
.BR sigreturn (2)).
.IP (3)
The kernel passes control back to user-space, where execution
commences at the start of the signal handler function.
.IP (4)
When the signal handler returns, control passes to the signal trampoline code.
.IP (5)
The signal trampoline calls
.BR sigreturn (2),
a system call that uses the information in the stack frame created in step 1
to restore the thread to its state before the signal handler was
called.
The thread's signal mask and alternate signal stack settings
are restored as part of this procedure.
Upon completion of the call to
.BR sigreturn (2),
the kernel transfers control back to user space,
and the thread recommences execution at the point where it was
interrupted by the signal handler.
.P
Note that if the signal handler does not return
(e.g., control is transferred out of the handler using
.BR siglongjmp (3),
or the handler executes a new program with
.BR execve (2)),
then the final step is not performed.
In particular, in such scenarios it is the programmer's responsibility
to restore the state of the signal mask (using
.BR sigprocmask (2)),
if it is desired to unblock the signals that were blocked on entry
to the signal handler.
(Note that
.BR siglongjmp (3)
may or may not restore the signal mask, depending on the
.I savesigs
value that was specified in the corresponding call to
.BR sigsetjmp (3).)
.P
From the kernel's point of view,
execution of the signal handler code is exactly the same as the execution
of any other user-space code.
That is to say, the kernel does not record any special state information
indicating that the thread is currently executing inside a signal handler.
All necessary state information is maintained in user-space registers
and the user-space stack.
The depth to which nested signal handlers may be invoked is thus
limited only by the user-space stack (and sensible software design!).
.\"
.SS Standard signals
Linux supports the standard signals listed below.
The second column of the table indicates which standard (if any)
specified the signal: "P1990" indicates that the signal is described
in the original POSIX.1-1990 standard;
"P2001" indicates that the signal was added in SUSv2 and POSIX.1-2001.
.TS
l c c l
____
lB c c l.
Signal	Standard	Action	Comment
SIGABRT	P1990	Core	Abort signal from \fBabort\fP(3)
SIGALRM	P1990	Term	Timer signal from \fBalarm\fP(2)
SIGBUS	P2001	Core	Bus error (bad memory access)
SIGCHLD	P1990	Ign	Child stopped or terminated
SIGCLD	\-	Ign	A synonym for \fBSIGCHLD\fP
SIGCONT	P1990	Cont	Continue if stopped
SIGEMT	\-	Term	Emulator trap
SIGFPE	P1990	Core	Floating-point exception
SIGHUP	P1990	Term	Hangup detected on controlling terminal
			or death of controlling process
SIGILL	P1990	Core	Illegal Instruction
SIGINFO	\-		A synonym for \fBSIGPWR\fP
SIGINT	P1990	Term	Interrupt from keyboard
SIGIO	\-	Term	I/O now possible (4.2BSD)
SIGIOT	\-	Core	IOT trap. A synonym for \fBSIGABRT\fP
SIGKILL	P1990	Term	Kill signal
SIGLOST	\-	Term	File lock lost (unused)
SIGPIPE	P1990	Term	Broken pipe: write to pipe with no
			readers; see \fBpipe\fP(7)
SIGPOLL	P2001	Term	Pollable event (Sys V);
			synonym for \fBSIGIO\fP
SIGPROF	P2001	Term	Profiling timer expired
SIGPWR	\-	Term	Power failure (System V)
SIGQUIT	P1990	Core	Quit from keyboard
SIGSEGV	P1990	Core	Invalid memory reference
SIGSTKFLT	\-	Term	Stack fault on coprocessor (unused)
SIGSTOP	P1990	Stop	Stop process
SIGTSTP	P1990	Stop	Stop typed at terminal
SIGSYS	P2001	Core	Bad system call (SVr4);
			see also \fBseccomp\fP(2)
SIGTERM	P1990	Term	Termination signal
SIGTRAP	P2001	Core	Trace/breakpoint trap
SIGTTIN	P1990	Stop	Terminal input for background process
SIGTTOU	P1990	Stop	Terminal output for background process
SIGUNUSED	\-	Core	Synonymous with \fBSIGSYS\fP
SIGURG	P2001	Ign	Urgent condition on socket (4.2BSD)
SIGUSR1	P1990	Term	User-defined signal 1
SIGUSR2	P1990	Term	User-defined signal 2
SIGVTALRM	P2001	Term	Virtual alarm clock (4.2BSD)
SIGXCPU	P2001	Core	CPU time limit exceeded (4.2BSD);
			see \fBsetrlimit\fP(2)
SIGXFSZ	P2001	Core	File size limit exceeded (4.2BSD);
			see \fBsetrlimit\fP(2)
SIGWINCH	\-	Ign	Window resize signal (4.3BSD, Sun)
.TE
.P
The signals
.B SIGKILL
and
.B SIGSTOP
cannot be caught, blocked, or ignored.
.P
Up to and including Linux 2.2, the default behavior for
.BR SIGSYS ", " SIGXCPU ", " SIGXFSZ ,
and (on architectures other than SPARC and MIPS)
.B SIGBUS
was to terminate the process (without a core dump).
(On some other UNIX systems the default action for
.BR SIGXCPU " and " SIGXFSZ
is to terminate the process without a core dump.)
Linux 2.4 conforms to the POSIX.1-2001 requirements for these signals,
terminating the process with a core dump.
.P
.B SIGEMT
is not specified in POSIX.1-2001, but nevertheless appears
on most other UNIX systems,
where its default action is typically to terminate
the process with a core dump.
.P
.B SIGPWR
(which is not specified in POSIX.1-2001) is typically ignored
by default on those other UNIX systems where it appears.
.P
.B SIGIO
(which is not specified in POSIX.1-2001) is ignored by default
on several other UNIX systems.
.\"
.SS Queueing and delivery semantics for standard signals
If multiple standard signals are pending for a process,
the order in which the signals are delivered is unspecified.
.P
Standard signals do not queue.
If multiple instances of a standard signal are generated while
that signal is blocked,
then only one instance of the signal is marked as pending
(and the signal will be delivered just once when it is unblocked).
In the case where a standard signal is already pending, the
.I siginfo_t
structure (see
.BR sigaction (2))
associated with that signal is not overwritten
on arrival of subsequent instances of the same signal.
Thus, the process will receive the information
associated with the first instance of the signal.
.\"
.SS Signal numbering for standard signals
The numeric value for each signal is given in the table below.
As shown in the table, many signals have different numeric values
on different architectures.
The first numeric value in each table row shows the signal number
on x86, ARM, and most other architectures;
the second value is for Alpha and SPARC; the third is for MIPS;
and the last is for PARISC.
A dash (\-) denotes that a signal is absent on the corresponding architecture.
.TS
l c c c c l
l c c c c l
______
lB c c c c l.
Signal	x86/ARM	Alpha/	MIPS	PARISC	Notes
	most others	SPARC
SIGHUP	\01	\01	\01	\01
SIGINT	\02	\02	\02	\02
SIGQUIT	\03	\03	\03	\03
SIGILL	\04	\04	\04	\04
SIGTRAP	\05	\05	\05	\05
SIGABRT	\06	\06	\06	\06
SIGIOT	\06	\06	\06	\06
SIGBUS	\07	10	10	10
SIGEMT	\-	\07	\07	-
SIGFPE	\08	\08	\08	\08
SIGKILL	\09	\09	\09	\09
SIGUSR1	10	30	16	16
SIGSEGV	11	11	11	11
SIGUSR2	12	31	17	17
SIGPIPE	13	13	13	13
SIGALRM	14	14	14	14
SIGTERM	15	15	15	15
SIGSTKFLT	16	\-	\-	\07
SIGCHLD	17	20	18	18
SIGCLD	\-	\-	18	\-
SIGCONT	18	19	25	26
SIGSTOP	19	17	23	24
SIGTSTP	20	18	24	25
SIGTTIN	21	21	26	27
SIGTTOU	22	22	27	28
SIGURG	23	16	21	29
SIGXCPU	24	24	30	12
SIGXFSZ	25	25	31	30
SIGVTALRM	26	26	28	20
SIGPROF	27	27	29	21
SIGWINCH	28	28	20	23
SIGIO	29	23	22	22
SIGPOLL					Same as SIGIO
SIGPWR	30	29/\-	19	19
SIGINFO	\-	29/\-	\-	\-
SIGLOST	\-	\-/29	\-	\-
SIGSYS	31	12	12	31
SIGUNUSED	31	\-	\-	31
.TE
.P
Note the following:
.IP \[bu] 3
Where defined,
.B SIGUNUSED
is synonymous with
.BR SIGSYS .
Since glibc 2.26,
.B SIGUNUSED
is no longer defined on any architecture.
.IP \[bu]
Signal 29 is
.BR SIGINFO / SIGPWR
(synonyms for the same value) on Alpha but
.B SIGLOST
on SPARC.
.\"
.SS Real-time signals
Starting with Linux 2.2,
Linux supports real-time signals as originally defined in the POSIX.1b
real-time extensions (and now included in POSIX.1-2001).
The range of supported real-time signals is defined by the macros
.B SIGRTMIN
and
.BR SIGRTMAX .
POSIX.1-2001 requires that an implementation support at least
.B _POSIX_RTSIG_MAX
(8) real-time signals.
.P
The Linux kernel supports a range of 33 different real-time
signals, numbered 32 to 64.
However, the glibc POSIX threads implementation internally uses
two (for NPTL) or three (for LinuxThreads) real-time signals
(see
.BR pthreads (7)),
and adjusts the value of
.B SIGRTMIN
suitably (to 34 or 35).
Because the range of available real-time signals varies according
to the glibc threading implementation (and this variation can occur
at run time according to the available kernel and glibc),
and indeed the range of real-time signals varies across UNIX systems,
programs should
.IR "never refer to real-time signals using hard-coded numbers" ,
but instead should always refer to real-time signals using the notation
.BR SIGRTMIN +n,
and include suitable (run-time) checks that
.BR SIGRTMIN +n
does not exceed
.BR SIGRTMAX .
.P
Unlike standard signals, real-time signals have no predefined meanings:
the entire set of real-time signals can be used for application-defined
purposes.
.P
The default action for an unhandled real-time signal is to terminate the
receiving process.
.P
Real-time signals are distinguished by the following:
.IP \[bu] 3
Multiple instances of real-time signals can be queued.
By contrast, if multiple instances of a standard signal are delivered
while that signal is currently blocked, then only one instance is queued.
.IP \[bu]
If the signal is sent using
.BR sigqueue (3),
an accompanying value (either an integer or a pointer) can be sent
with the signal.
If the receiving process establishes a handler for this signal using the
.B SA_SIGINFO
flag to
.BR sigaction (2),
then it can obtain this data via the
.I si_value
field of the
.I siginfo_t
structure passed as the second argument to the handler.
Furthermore, the
.I si_pid
and
.I si_uid
fields of this structure can be used to obtain the PID
and real user ID of the process sending the signal.
.IP \[bu]
Real-time signals are delivered in a guaranteed order.
Multiple real-time signals of the same type are delivered in the order
they were sent.
If different real-time signals are sent to a process, they are delivered
starting with the lowest-numbered signal.
(I.e., low-numbered signals have highest priority.)
By contrast, if multiple standard signals are pending for a process,
the order in which they are delivered is unspecified.
.P
If both standard and real-time signals are pending for a process,
POSIX leaves it unspecified which is delivered first.
Linux, like many other implementations, gives priority
to standard signals in this case.
.P
According to POSIX, an implementation should permit at least
.B _POSIX_SIGQUEUE_MAX
(32) real-time signals to be queued to
a process.
However, Linux does things differently.
Up to and including Linux 2.6.7, Linux imposes
a system-wide limit on the number of queued real-time signals
for all processes.
This limit can be viewed and (with privilege) changed via the
.I /proc/sys/kernel/rtsig\-max
file.
A related file,
.IR /proc/sys/kernel/rtsig\-nr ,
can be used to find out how many real-time signals are currently queued.
In Linux 2.6.8, these
.I /proc
interfaces were replaced by the
.B RLIMIT_SIGPENDING
resource limit, which specifies a per-user limit for queued
signals; see
.BR setrlimit (2)
for further details.
.P
The addition of real-time signals required the widening
of the signal set structure
.RI ( sigset_t )
from 32 to 64 bits.
Consequently, various system calls were superseded by new system calls
that supported the larger signal sets.
The old and new system calls are as follows:
.TS
lb lb
l l.
Linux 2.0 and earlier	Linux 2.2 and later
\fBsigaction\fP(2)	\fBrt_sigaction\fP(2)
\fBsigpending\fP(2)	\fBrt_sigpending\fP(2)
\fBsigprocmask\fP(2)	\fBrt_sigprocmask\fP(2)
\fBsigreturn\fP(2)	\fBrt_sigreturn\fP(2)
\fBsigsuspend\fP(2)	\fBrt_sigsuspend\fP(2)
\fBsigtimedwait\fP(2)	\fBrt_sigtimedwait\fP(2)
.TE
.\"
.SS Interruption of system calls and library functions by signal handlers
If a signal handler is invoked while a system call or library
function call is blocked, then either:
.IP \[bu] 3
the call is automatically restarted after the signal handler returns; or
.IP \[bu]
the call fails with the error
.BR EINTR .
.P
Which of these two behaviors occurs depends on the interface and
whether or not the signal handler was established using the
.B SA_RESTART
flag (see
.BR sigaction (2)).
The details vary across UNIX systems;
below, the details for Linux.
.P
If a blocked call to one of the following interfaces is interrupted
by a signal handler, then the call is automatically restarted
after the signal handler returns if the
.B SA_RESTART
flag was used; otherwise the call fails with the error
.BR EINTR :
.\" The following system calls use ERESTARTSYS,
.\" so that they are restartable
.IP \[bu] 3
.BR read (2),
.BR readv (2),
.BR write (2),
.BR writev (2),
and
.BR ioctl (2)
calls on "slow" devices.
A "slow" device is one where the I/O call may block for an
indefinite time, for example, a terminal, pipe, or socket.
If an I/O call on a slow device has already transferred some
data by the time it is interrupted by a signal handler,
then the call will return a success status
(normally, the number of bytes transferred).
Note that a (local) disk is not a slow device according to this definition;
I/O operations on disk devices are not interrupted by signals.
.IP \[bu]
.BR open (2),
if it can block (e.g., when opening a FIFO; see
.BR fifo (7)).
.IP \[bu]
.BR wait (2),
.BR wait3 (2),
.BR wait4 (2),
.BR waitid (2),
and
.BR waitpid (2).
.IP \[bu]
Socket interfaces:
.\" If a timeout (setsockopt()) is in effect on the socket, then these
.\" system calls switch to using EINTR.  Consequently, they and are not
.\" automatically restarted, and they show the stop/cont behavior
.\" described below.  (Verified from Linux 2.6.26 source, and by experiment; mtk)
.BR accept (2),
.BR connect (2),
.BR recv (2),
.BR recvfrom (2),
.BR recvmmsg (2),
.BR recvmsg (2),
.BR send (2),
.BR sendto (2),
and
.BR sendmsg (2),
.\" FIXME What about sendmmsg()?
unless a timeout has been set on the socket (see below).
.IP \[bu]
File locking interfaces:
.BR flock (2)
and
the
.B F_SETLKW
and
.B F_OFD_SETLKW
operations of
.BR fcntl (2)
.IP \[bu]
POSIX message queue interfaces:
.BR mq_receive (3),
.BR mq_timedreceive (3),
.BR mq_send (3),
and
.BR mq_timedsend (3).
.IP \[bu]
.BR futex (2)
.B FUTEX_WAIT
(since Linux 2.6.22;
.\" commit 72c1bbf308c75a136803d2d76d0e18258be14c7a
beforehand, always failed with
.BR EINTR ).
.IP \[bu]
.BR getrandom (2).
.IP \[bu]
.BR pthread_mutex_lock (3),
.BR pthread_cond_wait (3),
and related APIs.
.IP \[bu]
.BR futex (2)
.BR FUTEX_WAIT_BITSET .
.IP \[bu]
POSIX semaphore interfaces:
.BR sem_wait (3)
and
.BR sem_timedwait (3)
(since Linux 2.6.22;
.\" as a consequence of the 2.6.22 changes in the futex() implementation
beforehand, always failed with
.BR EINTR ).
.IP \[bu]
.BR read (2)
from an
.BR inotify (7)
file descriptor
(since Linux 3.8;
.\" commit 1ca39ab9d21ac93f94b9e3eb364ea9a5cf2aba06
beforehand, always failed with
.BR EINTR ).
.P
The following interfaces are never restarted after
being interrupted by a signal handler,
regardless of the use of
.BR SA_RESTART ;
they always fail with the error
.B EINTR
when interrupted by a signal handler:
.\" These are the system calls that give EINTR or ERESTARTNOHAND
.\" on interruption by a signal handler.
.IP \[bu] 3
"Input" socket interfaces, when a timeout
.RB ( SO_RCVTIMEO )
has been set on the socket using
.BR setsockopt (2):
.BR accept (2),
.BR recv (2),
.BR recvfrom (2),
.BR recvmmsg (2)
(also with a non-NULL
.I timeout
argument),
and
.BR recvmsg (2).
.IP \[bu]
"Output" socket interfaces, when a timeout
.RB ( SO_RCVTIMEO )
has been set on the socket using
.BR setsockopt (2):
.BR connect (2),
.BR send (2),
.BR sendto (2),
and
.BR sendmsg (2).
.\" FIXME What about sendmmsg()?
.IP \[bu]
Interfaces used to wait for signals:
.BR pause (2),
.BR sigsuspend (2),
.BR sigtimedwait (2),
and
.BR sigwaitinfo (2).
.IP \[bu]
File descriptor multiplexing interfaces:
.BR epoll_wait (2),
.BR epoll_pwait (2),
.BR poll (2),
.BR ppoll (2),
.BR select (2),
and
.BR pselect (2).
.IP \[bu]
System V IPC interfaces:
.\" On some other systems, SA_RESTART does restart these system calls
.BR msgrcv (2),
.BR msgsnd (2),
.BR semop (2),
and
.BR semtimedop (2).
.IP \[bu]
Sleep interfaces:
.BR clock_nanosleep (2),
.BR nanosleep (2),
and
.BR usleep (3).
.IP \[bu]
.BR io_getevents (2).
.P
The
.BR sleep (3)
function is also never restarted if interrupted by a handler,
but gives a success return: the number of seconds remaining to sleep.
.P
In certain circumstances, the
.BR seccomp (2)
user-space notification feature can lead to restarting of system calls
that would otherwise never be restarted by
.BR SA_RESTART ;
for details, see
.BR seccomp_unotify (2).
.\"
.SS Interruption of system calls and library functions by stop signals
On Linux, even in the absence of signal handlers,
certain blocking interfaces can fail with the error
.B EINTR
after the process is stopped by one of the stop signals
and then resumed via
.BR SIGCONT .
This behavior is not sanctioned by POSIX.1, and doesn't occur
on other systems.
.P
The Linux interfaces that display this behavior are:
.IP \[bu] 3
"Input" socket interfaces, when a timeout
.RB ( SO_RCVTIMEO )
has been set on the socket using
.BR setsockopt (2):
.BR accept (2),
.BR recv (2),
.BR recvfrom (2),
.BR recvmmsg (2)
(also with a non-NULL
.I timeout
argument),
and
.BR recvmsg (2).
.IP \[bu]
"Output" socket interfaces, when a timeout
.RB ( SO_RCVTIMEO )
has been set on the socket using
.BR setsockopt (2):
.BR connect (2),
.BR send (2),
.BR sendto (2),
and
.\" FIXME What about sendmmsg()?
.BR sendmsg (2),
if a send timeout
.RB ( SO_SNDTIMEO )
has been set.
.IP \[bu]
.BR epoll_wait (2),
.BR epoll_pwait (2).
.IP \[bu]
.BR semop (2),
.BR semtimedop (2).
.IP \[bu]
.BR sigtimedwait (2),
.BR sigwaitinfo (2).
.IP \[bu]
Linux 3.7 and earlier:
.BR read (2)
from an
.BR inotify (7)
file descriptor
.\" commit 1ca39ab9d21ac93f94b9e3eb364ea9a5cf2aba06
.IP \[bu]
Linux 2.6.21 and earlier:
.BR futex (2)
.BR FUTEX_WAIT ,
.BR sem_timedwait (3),
.BR sem_wait (3).
.IP \[bu]
Linux 2.6.8 and earlier:
.BR msgrcv (2),
.BR msgsnd (2).
.IP \[bu]
Linux 2.4 and earlier:
.BR nanosleep (2).
.SH STANDARDS
POSIX.1, except as noted.
.SH NOTES
For a discussion of async-signal-safe functions, see
.BR signal\-safety (7).
.P
The
.IR /proc/ pid /task/ tid /status
file contains various fields that show the signals
that a thread is blocking
.RI ( SigBlk ),
catching
.RI ( SigCgt ),
or ignoring
.RI ( SigIgn ).
(The set of signals that are caught or ignored will be the same
across all threads in a process.)
Other fields show the set of pending signals that are directed to the thread
.RI ( SigPnd )
as well as the set of pending signals that are directed
to the process as a whole
.RI ( ShdPnd ).
The corresponding fields in
.IR /proc/ pid /status
show the information for the main thread.
See
.BR proc (5)
for further details.
.SH BUGS
There are six signals that can be delivered
as a consequence of a hardware exception:
.BR SIGBUS ,
.BR SIGEMT ,
.BR SIGFPE ,
.BR SIGILL ,
.BR SIGSEGV ,
and
.BR SIGTRAP .
Which of these signals is delivered,
for any given hardware exception,
is not documented and does not always make sense.
.P
For example, an invalid memory access that causes delivery of
.B SIGSEGV
on one CPU architecture may cause delivery of
.B SIGBUS
on another architecture, or vice versa.
.P
For another example, using the x86
.I int
instruction with a forbidden argument
(any number other than 3 or 128)
causes delivery of
.BR SIGSEGV ,
even though
.B SIGILL
would make more sense,
because of how the CPU reports the forbidden operation to the kernel.
.SH SEE ALSO
.BR kill (1),
.BR clone (2),
.BR getrlimit (2),
.BR kill (2),
.BR pidfd_send_signal (2),
.BR restart_syscall (2),
.BR rt_sigqueueinfo (2),
.BR setitimer (2),
.BR setrlimit (2),
.BR sgetmask (2),
.BR sigaction (2),
.BR sigaltstack (2),
.BR signal (2),
.BR signalfd (2),
.BR sigpending (2),
.BR sigprocmask (2),
.BR sigreturn (2),
.BR sigsuspend (2),
.BR sigwaitinfo (2),
.BR abort (3),
.BR bsd_signal (3),
.BR killpg (3),
.BR longjmp (3),
.BR pthread_sigqueue (3),
.BR raise (3),
.BR sigqueue (3),
.BR sigset (3),
.BR sigsetops (3),
.BR sigvec (3),
.BR sigwait (3),
.BR strsignal (3),
.BR swapcontext (3),
.BR sysv_signal (3),
.BR core (5),
.BR proc (5),
.BR nptl (7),
.BR pthreads (7),
.BR sigevent (3type)
